Conventionally, as an optical fiber sensor applied to a light sensing technology, an OTDR (Optical Time Domain Reflectrometer)-based optical fiber sensor which measures the physical quantity (such as temperature and loss) and a defect position in an optical fiber, by measuring a backscattering light of Rayleigh scattering which is generated when a light pulse is inputted into the optical fiber, has been known.
On the other hand, the shape of Brillouin gain spectrum (BGS) of Brillouin scattering light output from an optical fiber changes due to temperature and/or strain of the optical fiber. As a technology to measure the physical quantity of an object (to be measured) by the use of this change, a BOCDA (Brillouin Optical Correlation Domain Analysis)-type optical fiber sensor described, for example, in patent document 1 and non-patent document 1 has been known.
The technology disclosed in the above-described patent document 1 and non-patent document 1, while modulating the frequencies of pumping light and probe light, inputting these pumping light and probe light oppositely from both ends of an optical fiber, and thereby causing Brillouin scattering to occur at a desired position in the longitudinal direction of the optical fiber. The temperature distribution along the longitudinal direction of the optical fiber is measured by measuring a gain spectrum resulting from this Brillouin scattering.
In accordance with this kind of optical fiber sensor with a fiber-optic distributed strain sensing technology (BOCDA-type) adopting Brillouin scattering method, by a continuous lightwave correlation control method, in comparison with the optical fiber sensing with Raman scattering method or the like that has been conventionally known, the superior effects in various characteristics such as a measurement temperature range, the spatial resolution of the temperature distribution measurement in the longitudinal direction, measuring time, and the like can be expected.
For example, a BOCDA-type optical fiber sensor can maintain a sufficient level of the intensity (gain) of Stokes light even at a very low temperature. Furthermore, because a BOCDA-type optical fiber sensor measures temperature by frequency shift, it has been confirmed that the optical fiber sensor is higher in noise tolerance than that in optical fiber sensing with Raman scattering method in which temperature is measured based on the light intensity ratio, and can measure down to the absolute temperature 1K.
Also, as described below, about the spatial resolution of a BOCDA-type optical fiber sensor, a typical optical fiber can realize a sampling interval of approximately 1 cm in the longitudinal direction. Furthermore, because continuous light is used in a BOCDA-type optical fiber sensor, it has been confirmed that it is possible to measure at a high speed, and it is possible to measure at 57 Hz per measurement point.
Furthermore, a BOCDA-type optical fiber sensor can adjust freely the spatial resolution, measuring range, and measuring time of the temperature distribution measurement along the longitudinal direction of an optical fiber, in accordance with the characteristics of an object to be measured and the like, by adjusting the frequency-modulation pattern of pumping light and probe light. Also, a BOCDA-type optical fiber sensor can adjust freely sampling intervals in addition to spatial resolution.    Patent document 1: Japanese Patent Laid-Open Publication No. 2000-180265    Non-patent document 1: Kazuo Hotate, and Hiroshi Arai, “Enlargement of Measurement Range by a Temporal Gating Scheme in BOCDA Fiber-Optic Distributed Strain Sensing System with Time-Division Pump-Probe Generation Scheme”, IEICE Technical Report, The Institute of Electronics, Information and Communication Engineers, OPE2004-224 (2005-02)